Unfortunately, most sites don’t publish their stats for public viewing so accurate numbers are hard to come by. At best, you can look for a website’s “advertising page” which may include marketing materials, demographic information, and yes, monthly traffic data.
But when that’s not available, your only option is to rely on a website traffic estimator.
Since these are never 100 percent accurate, I only recommend using them to compare the traffic of sites in relative terms—and even then, you should only compare readings from the same tool.
SimilarWeb is my estimator of choice when I want to see what kind of traffic a website gets.
The real value of SimilarWeb is its top website ranking page where you can the top ranking sites according to category and country (limited to the top 50 for free users), but it also lets you search for a specific domain and see that particular site’s stats.
When you look up a site, you get three points of data right away: global rank, country rank, and category rank. These are awesome for website competition at a glance. But if you scroll down, you can see the engagement stats: monthly traffic, average visit duration, pages per visit, and bounce rate.
Quantcast is probably the most accurate traffic estimator tool currently available, but it comes with two important caveats: first, its accuracy is still spotty from site to site, and second, its data set is severely limited when compared to sites like SimilarWeb or Alexa.
Lastly, SEMRush is primarily a search engine optimization tool, meaning you’d use it as a website owner to help find and target keywords that bring you more search engine traffic. However, as a regular web surfer, you can use it to see what kind of search traffic a site gets. If you find out that you are way behind your competition, then I would recommend a quality traffic tool called Babylon.